1. Field of the Invention
This invention relates to a mask pattern generation method.
2. Description of the Related Art
In a lithography technique in a process of manufacturing a semiconductor device, an exposure apparatus including an illumination optical system for illuminating a mask (a reticle) with light from a light source and a projection optical system for projecting an image of a pattern of a mask onto a substrate (e.g., a wafer) is used.
As a minimum size of a target pattern to be formed on the substrate becomes a size lower than the wavelength of the light from the light source to be used for exposure, when the mask pattern image is projected onto the substrate, unintended interactions occur between adjacent patterns. The interactions of the light from respective patterns in the mask cause formation of an unintended image of a shape different from the target pattern on the substrate. With the increase in the difference between the minimum size of the target pattern and the wavelength of the light source, the occurrence of the resolution failure of the pattern increases.
There has been known a method for performing an optical proximity correction (OPC) on the pattern of the mask to decrease such the resolution failure. In the OPC, in consideration of the effects on the image of the pattern due to the interactions of the light from adjacent patterns, a correction for changing the shape of the pattern of the mask is performed so that the image of the pattern is formed within a target range.
Japanese Patent Laid-Open No. 2011-095729 discloses performing Source Mask Optimization (SMO) which is optimization of both an illumination mode for illuminating the mask (a light intensity distribution on a pupil of the illumination optical system) and a shape of the pattern of the mask using a computer. Data of the pattern of the mask is represented by GDS format for example. A figure given by apexes of a polygon as a design value is treated as a parameter. In the SMO, an evaluation position of the image of the pattern is determined, an evaluation result at the evaluation position is fed back to the variable.
Japanese Patent Laid-Open No. 2005-181636 discloses that the pattern of the mask is treated as a parameter, and the evaluation position of the image of the pattern is set at a midpoint between two apexes of a polygon.
On the other hand, due to, for example, a reduction in the factor k1, it has been becoming difficult to transfer a desired pattern onto a wafer with high fidelity using the conventional two-dimensional layout pattern (that extends in vertical and horizontal directions). Therefore, in recent years, a method for manufacturing a circuit pattern that is called the one-dimensional layout technique has been contrived as shown in Michael C. Smayling et. al., “Low k1 Logic Design using Gridded Design Rules” Proc. of SPIE Vol. 6925 p. 69250B (2008). According to the one-dimensional layout technique, a line and space (L/S) pattern based on a single pitch is formed. After that, a plurality of pattern elements such as a cut pattern, which has equal image dimensions, is transferred onto a same grid at a plurality of positions by an exposure. In this manner, a circuit pattern is fabricated by cutting the L/S pattern based on the single pitch, by the plurality of pattern elements. This method can not only reduce an exposed area compared to the conventional two-dimensional pattern, but also make resolution of the pattern technically easier.
In designing the cut pattern of the one-dimensional layout, due to design procedure, a plurality of pattern elements is arranged to overlap or contact with each other for cutting neighboring lines.
In the case that the polygons overlap or contact with each other, the evaluation position is also set on a side at which the polygons overlap or contact with each other by use of the method disclosed in Japanese Patent Laid-Open No. 2005-181636. However, this evaluation position is inappropriate for evaluating the image of the pattern. When calculation of the optimization is performed with setting the evaluation position like this, the calculation time becomes longer, and an inappropriate calculation result is output.
FIGS. 1A to 1C are examples of a case that the evaluation position or parameter is set by use of the method disclosed in Japanese Patent Laid-Open No. 2005-181636 when the polygons contact with each other. FIG. 1A shows respective sides (edges) of a polygon 101 and 201 as targets. A case that a lower side 103 of the polygon 101 contacts an upper side 202 of the polygon 201 as shown in FIG. 1B is described below.
FIG. 1B shows parameters of polygons. A parameter for adjusting width of the polygon 101 in the longitudinal direction is represented by a parameter 110 of width between an upper side 102 and a lower side 103. A parameter for adjusting width of the polygon 101 in the lateral direction is represented by a parameter 111 of width between a right side 104 and a left side 105. A parameter for adjusting width of the polygon 201 in the longitudinal direction is represented by a parameter 210 of width between an upper side 202 and a lower side 203. A parameter for adjusting width of the polygon 201 in the lateral direction is represented by a parameter 211 of width between a right side 204 and a left side 205.
The evaluation position is set at the center between apexes of each polygon. Therefore, evaluation positions 301 to 307 are set on respective sides of the polygons as shown in FIG. 1C. Here, an evaluation position 302 set on the side 103 (202) is focused. An image of the polygon 101 and the polygon 201 formed on the substrate (an image plane) is one successive image because the polygon 101 and the polygon 201 are contacted along the side 103 and the side 202. Therefore, it is impossible to calculate width of an image between the evaluation positions 301 and 302 and width of an image between the evaluation positions 302 and 303 when an imaging evaluation is performed. Hence, values of the parameters 110 or 210 become error. There may an issue when optimization calculation is performed, an optimized value of the parameter is not obtained.